One can be a bit untidy
I believe the answer is:
unit
'one' is the definition.
(synonyms)
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for unit that I've seen before include "Body of soldiers" , "Individual thing; one" , "Single item, maybe part of a set" , "Single element; one" , "A single, complete item" .)